The present invention relates to a driving circuit for selectively driving a plurality of driven elements, such as light-emitting diodes used as light sources in an electrophotographic printer, heating elements in a thermal printer, or display elements in a display device.
In the following description, the driven elements are be light-emitting diodes or LEDs employed in an electrophotographic printer.
In a conventional electrophotographic printer, for example, an electrically charged photosensitive drum is selectively illuminated, responsive to the data to be printed, to form a latent electrostatic image, which is developed by application of toner particles to form a toner image. The toner image is then transferred to paper and fused onto the paper.
FIG. 15 is a block diagram of the control circuitry of a conventional electrophotographic printer. FIG. 16 is a timing diagram illustrating the operation of the conventional electrophotographic printer.
The printing control unit 1 in FIG. 15 comprises a microprocessor, read-only memory, random-access memory, input-output ports, timers, and other elements disposed in the printing engine of the printer. The printing control unit 1 receives a control signal SG1 and a dot data signal SG2 from a higher-order controller, and controls the printing operations performed by the printing engine. The dot data signal SG2 is a one-dimensional digital signal representing a two-dimensional bit map of picture elements (pixels), referred to below as dots.
Upon receiving a print command via control signal SG1, the printing control unit 1 first checks a temperature sensor 23 to determine whether the fuser 22 is within the necessary temperature range. If the fuser 22 is not within the necessary temperature range, the printing control unit 1 activates a heater 22a built into the fuser 22. When the fuser 22 reaches the necessary temperature, the printing control unit 1 activates a driver 2 that drives a stepping motor or pulse motor (PM) 3 used in the developing and transfer process, and activates a charge signal SGC that switches on a high-voltage power source 25 that charges toner particles in a developer unit 27.
The presence or absence of paper and the size of the paper are detected by a paper sensor 8 and size sensor 9. If paper is present, the printing control unit 1 activates a driver 4 that drives another pulse motor (PM) 5. This motor is first driven in reverse by a certain amount, until paper is detected by a pick-up sensor 6, then driven forward to feed the paper into the printing engine.
When the paper has been fed to the necessary position, the printing control unit 1 sends timing signals SG3 (including horizontal and vertical synchronization signals) to the higher-order controller, and begins receiving the dot data signal SG2, which the higher-order controller generates on a page-at-a-time basis. The dot data signal SG2 is supplied as a data signal HD-DATA to an LED head 19 comprising a row of LEDs, with one LED per dot. The transfer of dot data into the LED head 19 is synchronized with a clock signal (HD-CLK).
After sufficient dot data (HD-DATA) for one horizontal dot line have been transferred into the LED head 19, the printing control unit 1 sends the LED head 19 a load signal (HD-LOAD), causing the dot data to be latched in the LED head 19. The LED head 19 can then print this line while receiving dot data for the next line.
The LED head 19 prints the line by illuminating a photosensitive drum (not visible) which has been precharged to a negative electrical potential. The potential level of illuminated dots rises, creating a latent dot image. The toner in the developer unit 27 is also charged to a negative potential, so toner particles are electrostatically attracted to the illuminated dots, creating a toner image.
The LEDs are turned on and off in synchronization with a strobe signal (HD-STB-N). FIG. 16 illustrates the timing of this signal and other signals mentioned above. The SG3 pulses shown at the top of FIG. 16 are horizontal synchronization pulses. FIG. 16 illustrates three successive line-printing cycles, for printing lines Nxe2x88x921, N, and N+1 (where N is an arbitrary integer).
Referring again to FIG. 15, to transfer the toner image to the paper, the printing control unit 1 activates a transfer signal SG4 that turns on a high-voltage power source 26, generating a high positive voltage in a transfer unit 28. As the paper travels through a narrow gap between the photosensitive drum and transfer unit 28, the toner image is transferred by electrostatic attraction to the paper.
The paper with the toner image is then transported to the fuser 22, which has been heated by the heater 22a. The heat fuses the toner to the paper, which then passes an exit sensor 7 and is ejected from the printer.
The printing control unit 1 controls these operations so that the high-voltage power source 26 is switched off except while the paper is traveling past the transfer unit 28, as detected by sensors 6 and 9. When the paper passes the exit sensor 7, the printing control unit 1 also switches off the high-voltage power source 25 of the developer unit 27, and stops the pulse motor (PM) 3 used in the developing and transfer process.
The above sequence is repeated for each page.
FIG. 17 shows the structure of the conventional LED head 19 in more detail. The dot data HD-DATA and clock signal HD-CLK are provided to a shift register comprising, for example, four thousand nine hundred ninety-two flip-flop circuits FF1, FF2, . . . , FF4992 (this number of flip-flop circuits is appropriate for printing on A4-size paper at six hundred dots per inch). When four thousand nine hundred ninety-two bits of dot data have been clocked into this shift register, the load signal HD-LOAD is activated, causing the bits to be stored in latches LT1, LT2, . . . , LT4992. When the strobe signal HD-STB-N is driven low, bits set to the high logic level (xe2x80x981xe2x80x99) turn on LEDs LD1, LD2, . . . , LD4992 by way of an inverter G0, pre-buffer circuits G1, G2, . . . , G4992, and p-channel metal-oxide-semiconductor (MOS) transistors Tr1, Tr2, . . . , Tr4992. The transistors Tr1, Tr2, . . . , Tr4992 are the driving elements that allow driving current to flow from the power supply (VDD) to the anodes of the LEDs LD1, LD2, . . . , LD4992.
In a printer employing the LED head in FIG. 17, all of the driven LEDs LD1, LD2, . . . , LD4992 are switched on for the same length of time, determined by the strobe signal HD-STB-N. Thus if these LEDs or the driving elements Tr1, Tr2, . . . , Tr4992 do not have perfectly uniform electrical properties, the dots will be unevenly illuminated. This will lead to differences in the sizes of the electrostatic dots in the latent image formed on the photosensitive drum, and differences in the sizes of the dots printed on the page.
Referring to FIG. 18, the LEDs are disposed on a plurality of LED array chips, which are coupled by bonding wires to integrated driver circuits, referred to below as driver ICs. In the example shown, there are twenty-six LED array chips (CHP1 to CHP26), each with one hundred ninety-two LEDs. Each LED is individually wire-bonded to an output terminal of the corresponding driver IC. The driver ICs (DRV1 to DRV26) are also coupled in a cascaded series to receive the dot data and control signals shown in FIG. 17.
FIG. 19 shows the internal structure of the first pre-buffer circuit (G1) in FIG. 17, comprising an AND gate AD1, a p-channel MOS transistor TP1, and an n-channel MOS transistor TN1. The other pre-buffer circuits G2 to G4992 are similar.
Each driver IC also has a control-voltage generating circuit 209. The control-voltage generating circuit 209 comprises an operational amplifier 100, a p-channel MOS transistor 101, and a resistor with resistance Rref. The output of the operational amplifier 100 is coupled to all of the pre-buffer circuits in the same driver IC, more specifically to the source terminal of the n-channel MOS transistor in each pre-buffer circuit, e.g., TN1 in pre-buffer circuit G1. The p-channel MOS transistor 101 has the same gate length as each of the driving elements Tr1, Tr2, . . . , Tr4992. The inverting input terminal of the operational amplifier 100 receives a reference voltage VREF from a voltage reference circuit (not visible). The operational amplifier 100, p-channel MOS transistor 101, and resistance Rref form a feedback circuit that sets the current Iref flowing through p-channel MOS transistor 101 to ground to a value determined by the reference voltage VREF and resistance Rref, independent of the supply voltage VDD.
In U.S. patent application Ser. No. 08/694,055, the present inventor proposed a driving apparatus with circuitry for efficiently compensating for chip-to-chip and dot-to-dot variations in the amount of light produced when each LED is driven. In U.S. patent application Ser. No. 09/083,065, the present inventor proposed a driving apparatus with further circuits that compensated for variations in optical output caused by temperature differences between the LED array chips. These two types of compensation circuits substantially eliminated the differences in the optical output characteristics of the individual LEDs, but it was subsequently found that under some printing conditions, slight printing irregularities still remained.
Further experiments showed that the remaining irregularities depended on the number of LEDs driven simultaneously by each driver IC in the LED head. The amount of optical energy to which each dot was exposed when only one dot was driven per chip differed from the amount of optical energy when a large number of dots were driven per chip, because of differences in the rise times of the driving current waveforms.
FIG. 20 shows examples of these rise times. The upper waveform in FIG. 20 is the waveform of the strobe signal (HD-STB-N). The lower waveforms (ID1) indicate the driving current supplied to a single LED when one, eight, thirty-two, ninety-six, and one hundred ninety-two LEDs in the same LED array chip are driven simultaneously. As the number of driven LEDs (dots) increases, the rise time of the driving-current waveform (ID1) increases markedly. As there is no great increase in the fall time, the increase in rise time significantly shortens the effective driving time of the LED. Consequently, when many LEDs are driven in the same LED array chip, each illuminated dot receives less light.
The difference in rise times arises for the following reason. When an LED such as LD1 in FIG. 19 is driven, it needs to be driven by a current flow that is limited and does not vary, even if the power-supply voltage VDD fluctuates. The driving element Tr1 thus needs to have a high output impedance and must operate with a constant-current characteristic. For these reasons, the driving element Tr1 must have a comparatively large gate length. To provide adequate driving current to the LED, Tr1 also has a comparatively large gate width. The large gate area of Tr1 creates a large gate-source capacitance between the gate electrode wiring pattern and the semiconductor substrate of the driver IC.
When the strobe signal HD-STB-N is inactive (high), the output of the inverter comprising transistors TP1 and TN1 is high (VDD), the source and gate of the driving element Tr1 are at the same potential (VDD), the gate-source capacitance of Tr1 is in a discharged state, and Tr1 is turned off. When a logic xe2x80x981xe2x80x99 is loaded from the shift register into latch LT1 and the strobe signal HD-STB-N is asserted (driven low), the output of the inverter comprising transistors TP1 and TN1 changes from VDD to the control voltage Vcontrol output by the control-voltage generating circuit 209. A transient current I1 flows from the power supply VDD to the output terminal of the operational amplifier 100, mainly through the capacitive coupling between the source and gate of the driving element Tr1, charging the gate-source capacitance of Tr1, thereby turning Tr1 on.
When only one dot per driver IC is driven, the transient charging current I1 is well within the current-sinking capability of the operational amplifier 100, despite the large gate-source capacitance of the driving element Tr1. When many dots are driven simultaneously, however, the total flow of transient charging current from the gate-source capacitances of all of the activated driving elements becomes large enough to be limited by the current-sinking capability of the operational amplifier 100. Due to this limit, as more LEDs are driven, less current is available to charge the gate-source capacitance of each driving element, the driving elements turn on more slowly, and the rise time of the driving current waveforms increases.
When the strobe signal HD-STB-N goes high, the voltage output by the inverter comprising TP1 and TN1 rises from Vcontrol to VDD. A transient current I0 now flows from VDD through transistor TP1 into the gate of the driving element Tr1, discharging the gate-source capacitance, and Tr1 turns off. The transient discharging currents I0 are not concentrated onto a single terminal with limited current-sinking capability, and are substantially the same regardless of the number of dots driven simultaneously.
Accordingly, when many dots are driven simultaneously, the charging currents I1 are limited, while the discharging currents I0 are not. The rise times of the current waveforms are therefore lengthened, with no compensating lengthening of the fall times. As a result, the illuminated dots receive less light, their diameter is decreased, less toner is transferred, and the printed dots are insufficiently dark.
A further problem is that when the strobe signal HD-STB-N goes high and the driven LEDs are turned off, the rapid fall of the driving current causes voltage noise at the power-supply leads of the driver ICs. The voltage noise xcex94V at a particular lead is given by the following equation, in which L is the lead inductance, xcex94i is the total change in driving current supplied through the lead, and xcex94t is the fall time of the current waveform.
xcex94V=Lxc3x97(xcex94i/xcex94t)
When many dots are driven, xcex94i becomes large, while as FIG. 20 indicates, xcex94t remains comparatively small. As a result, the voltage noise xcex94V can become large enough to impair other operations in the driver IC. In the worst case, the driver IC can be damaged by the voltage noise.
It is accordingly an object of the present invention to supply driving current to a plurality of driven elements, at driving conditions that do not vary depending on the number of elements driven simultaneously.
Another object of the invention is to reduce voltage noise when the current supplied to the driven elements is switched off.
Another object is to provide an LED head in which the above objects are achieved.
According to a first aspect of the invention, each driven element is driven by a first driving element and a second driving element coupled in series. The first driving element sets the driving current to a predetermined value. The second driving element switches the driving current on and off.
In the first aspect, the first driving element operates statically. A substantially unlimited number of first driving elements can be coupled to the same control-voltage generating circuit without stressing the output capabilities of the control-voltage generating circuit.
According to a second aspect of the invention, a plurality of driven elements are driven by respective driving elements, which are switched on and off by respective pre-stage circuits. The pre-stage circuits are all coupled to a single control-voltage generating circuit, which supplies a control voltage used for switching the driving elements on. A resistor is inserted in series between each driving element and its pre-stage circuit.
In the second aspect, the resistors limit transient current flow to a value within the output capability of the control-voltage generating circuit, even when all of the driving elements are switched on simultaneously. The resistors also reduce voltage noise by making the driving elements turn off relatively slowly.
According to a third aspect of the invention, a plurality of driven elements are driven by respective driving elements, which are switched on and off by respective pre-stage circuits, all coupled to a single control-voltage generating circuit as in the second aspect of the invention. The pre-stage circuits operate with constant-current output characteristics.
In the third aspect, owing to the constant-current characteristics of the pre-stage circuits, the control-voltage generating circuit can be designed to provide the necessary output current, even when all driven elements are driven simultaneously. The constant-current characteristics can also be set so that the driven elements turn off slowly, thereby reducing voltage noise.